Downstream of man-made hydraulic structures, bed protections made of granular material are often used to prevent erosion of the bottom. The stones in the top layer of such granular filters must be able to withstand the hydraulic forces. Probably the best-known formula that is used to determine the stability of granular bed material is the design criterion by Shields (1936). However, this criterion is only valid for uniform flows. Little is known about the influence of turbulence on the stability of stones. In this thesis the influence of turbulence structures on the stability of stones in the top layer of a granular filter in one type of nonuniform flow that is often encountered near hydraulic structures, namely backward-facing step (BFS) flow, was investigated.